Violent and offensive images have swamped the newsfeeds of Facebook users, according to reports.
In the past 24 hours, the disturbing content has spread throughout the site without any knowledge of users, and there is little information on why.
The content consists of images of hardcore porn, a photoshopped celebrity in a sexual position, extreme violence and abused animals.
Facebook users took to Twitter and some vented their frustration claiming they may deactivate their account.
"I sign into Facebook and the first thing I see is a dead dog. My relationship with Facebook is now over," said Twitter user @KryshaBravo.
"#facebook is now #pornbook," added @AmoyBerry.
The content may have started to spread from users simply liking an application. In the past, scams have spread through clickjacking, where users are duped into liking content.
"Facebook is aware of these reports and we are investigating the issue," a Facebook spokesperson told IT Pro.
According to the social network, its "Facebook Immune System" (FIS) detects and stops harmful threats while protecting users. FIS learns, adapts and protects in the same ways a biological immune systems works in the body, according to Facebook.
However, the system relies on users doing their bit too by not clicking on suspicious links. It appears this guidance may have been disregarded in this case.
"In the end, security is a partnership between Facebook and the people who use it. Together we win," Facebook said.
A guide to Facebook security, by Facebook